The present invention relates to detecting the end of an etching process, and particularly to detecting the end of a plasma etching process.
Monitoring an etch process to determine the etch endpoint is a critical aspect of manufacturing semiconductor devices. Determining when an etch process has completely etched through one layer provides essential information regarding when to stop the etching process. Without an accurate determination of when a layer is completely etched through, an etch process may be terminated early, or proceed for too long, resulting in under-etching or over-etching of a semiconductor device.
Under-etching degrades a semiconductor device by leaving residues of the material etched through. For example, under-etching an insulator from a metal layer leaves insulator residue on the exposed metal, potentially causing poor vertical conduction between metal layers. Such unetched residue reduces the manufacturing yield of usable semiconductor devices from a wafer and increases the costs of manufacturing semiconductor devices.
Over-etching degrades a semiconductor device by attacking sidewalls and/or causing pattern degradation. In the case of etching a metal layer, over-etching may reduce conductor line width when the metal layer sidewalls are etched from underneath the mask, and/or could reduce the metal film thickness. Such reduced metal line width or reduced film thickness increases resistance and current density in the metal line during use of the semiconductor device and contributes to circuit failure. In the event contacts, e.g. vias, are over-etched, over-etching could increase the surface roughness of the contacts, or create undesirable undercut, or change the profile of the contact, which could adversely impact the deposition properties of the conductor.
Many methods have been developed and are utilized for detecting etch endpoints during semiconductor device manufacture. Commonly used methods determine an etch endpoint by monitoring a particular property of either the etching process parameters, or of the materials comprising the etch layer and layer underlying the etch layer. Because there are a variety of etching processes using different plasmas and other etch chemistries that etch through many different materials, a single etch endpoint determining method does not completely and accurately determine the end of all etching processes.
One technique for determining the end of a plasma etch is to time the etching process, and compare the elapsed time to a predetermined etch rate. A predetermined etch rate is experimentally determined in a calibration step, however, the exact conditions i.e., pressure, gas flow, electric field, etc., existing during the calibration step cannot be precisely duplicated when the etch is performed, rendering endpoint determination based upon elapsed time inaccurate.
More precise methods of determining the end of a plasma etch process analyze the by-products of the etching process, i.e., the plasma in the reaction chamber or the gas phase species within the plasma produced by reacting the plasma with the material being etched. Such techniques rely upon the fact that the initial plasma is a known composition, and that it is known what elements and compounds are in the material to be etched. Typically, a detector capable of detecting an element or compound released from the top layer, i.e., the layer to be etched, and/or the underlying layer is used to monitor the presence of such elemental species in the plasma, and sends a signal to diagnostic electronics. When the signal representing the presence of the monitored element in the plasma changes, the diagnostic electronics indicate that the etching process is complete.
One such technique, optical emission spectroscopy, monitors the intensity of optical emission in the plasma. The intensity of the optical emission is related to the concentration of elements and compounds in the plasma. The completion of the etch process is determined when a change in the intensity of the optical emission is observed, such as when the concentration of elements and/or compounds in the plasma changes as a result of etching through the top layer and into the underlying substrate.
Other by-product diagnostic techniques have been utilized to identify the endpoint of an etch process. These include laser interferometry, ellipsometry, acoustic monitoring (passing the gas stream from a reaction chamber of a plasma reactor through an acoustic cell at constant temperature to monitor the velocity of an acoustic signal which changes as the average molecular weight of the gas changes), monitoring a wafer using a charge coupled device camera which transduces reflected light into electromagnetic signals, and mass spectrometry.
A significant limitation to using by-product diagnostic techniques is an inherent lag time. Etching through the top layer is completed before there are detectable changes to the plasma that signify an etch endpoint. The amount of time that elapses between complete etching and detection of a change in the plasma increases the potential for damaging over-etching compared to a more rapid and real-time determination of the etch endpoint.
Another significant limitation to using by-product diagnostic techniques is that often a minute amount of material is etched, i.e., less than 1% of a wafer, thus only a minor change to the plasma and/or by-products occurs which can be very difficult to detect, sometimes resulting in failure to identify the end of the etching process. Etch endpoint determination is also difficult when a large amount of material is etched, i.e., a sputter etch to remove an oxide layer from a metal layer prior to deposition. Often, the volume of the chamber in which the etch is performed is too large to mount current etch endpoint determining equipment, and too large to adequately monitor etch by-products, leaving timing as the only manner for determining the etch endpoint.
Sputter etching typically occurs in a large volume chamber, which is in turn part of an even larger chamber. Because the volume of the chamber in which sputter etching occurs is large, current etch endpoint detection equipment cannot be mounted within the chamber in order to detect a change in the by-products of the etching process. Additionally, if spectroscopy instruments were mounted inside a sputter etch chamber, for example, part of a metal deposition system, the volume of system is too large for the equipment to accurately determine the composition of the etch by-products. Such difficulties resulting from the large volume in which sputter etching typically occurs currently makes. timing a large volume sputter etch process the only method for determining the etch endpoint. As previously noted, timing an etch process in order to determine an endpoint does not result in an accurate etch endpoint determination.
As manufacturing techniques adopt smaller device geometries, precise etching is becoming more critical to producing a high yield of working semiconductor devices from each wafer. The prior by-product techniques are suitable for detecting an etch endpoint in many circumstances, but do not adequately determine etch endpoints in many other circumstances as previously described.
Accordingly, there is a need for improved techniques for determining etch endpoints that are currently undetectable and/or detected well after the etch process is complete. There is a particular need to monitor currently unmonitored properties of an etch process in order to increase the number of different etch processes in which accurate endpoint determinations can be made.
There is a need for increasing the number of situations in which an etch endpoint can be determined. There is also a need for more accurately determining when an etch endpoint is reached.
These needs and others are met by embodiments of the present invention, which provide a method and apparatus for identifying an etch endpoint based upon reflected and secondary ions. An ion source is used to direct a beam of ions towards a wafer, and a detector is used to detect the reflected and secondary ions that scatter from the wafer as a result of the ion beam impacting the wafer. A change in the detected value of the reflected and scattered ions indicates an etching process endpoint has been reached. Embodiments of the present invention utilize a voltage contrast scanning electron microscope as a source of ions and as a detector of reflected and secondary ions.
Accordingly, one aspect of the invention relates to an apparatus for detecting an endpoint for an etch process. The apparatus comprises an ion source and an ion detector that emit ions and detect ions, respectively, in a reaction chamber. A signal processor is coupled to the detector. The ion source directs a beam of ions toward a wafer in the reaction chamber. The detector detects reflected primary ions and secondary ions that are scattered from the wafer as a result of the ion beam striking the wafer. Based upon the detected ions, the ion detector generates a signal. The signal processor determines a value based upon the signal from the detector.
In certain embodiments, a plasma source creates a plasma at a first frequency in the reaction chamber. A positive electrode in the reaction chamber contains the plasma on one side. The ion source and the ion detector are both shielded from the plasma by the positive electrode. A first aperture in the positive electrode allows the ion source to direct its beam of ions toward the wafer. A second aperture in the positive electrode allows the detector to detect the reflected and back scattered ions. The ions emitted by the ion source are at a second frequency that is different than the frequency of the ions in the plasma. A filter is coupled to the detector and to the signal processor, and removes the signal portions generated by detected ions at the first frequency, and transmits the filtered signal to the signal processor.
Another aspect of the present invention relates to a method for detecting an endpoint of an etch process. The method comprises the steps of emitting ions from a source towards a wafer; and detecting the reflected and secondary scattered ions released from the wafer as a result of the emitted ions from the ion source impacting the wafer. The method also comprises the steps of creating a signal based upon the detected reflected primary ions and scattered secondary ions; and determining a value for the detected ions based upon the signal. The method comprises the steps of etching the wafer; monitoring the value while the wafer is etched; and indicating the etch endpoint when the value changes to a second value.
In certain embodiments, the etch process is a plasma etch utilizing a plasma at a first frequency. Ions emitted from the ion source are at a second frequency; and determining the value of the detected ions further comprises filtering the signal to remove signal portions generated by detected ions at the first frequency.
Another aspect of the present invention relates to a metal deposition system comprising a vacuum chamber and an etch station within the vacuum chamber. The metal deposition system comprises a plasma source at the etch station, an ion source at the etch station, a detector at the etch station, and a signal processor communicating with the detector. The detector detects reflected primary ions and secondary ions scattered off of an oxide layer as a result of ions emitted from the ion source impacting the oxide layer. The detector also generates a signal based upon the detected reflected and scattered secondary ions. The signal processor determines a value based upon the signal from the detector.
Using an ion source to direct a beam of ions towards a wafer, and using a detector to detect the reflected and secondary ions that are scattered off the wafer as a result of the ion beam impacting the wafer, greatly enhances etch endpoint detection. By detecting a change in the value of the reflected and secondary scattered ions, a method for accurately and timely determining an etch endpoint is provided that does not rely on detection of etch by-products.
Additional advantages and novel features of the invention will be set forth in part by the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The advantages of the invention may be realized and attained by the instrumentalities and combinations, particularly pointed out in the appended claims.